Government and private officials often have responsibility for controlled areas subject to restrictive access to authorized persons. Such officials may employ various techniques to detect human presence. These tools generally depend on human activity to present a detectable signal.
Human activity may trigger a sensor based on various stimuli. For example, skeletal-muscular physical motion may form pressure gradients in the local environment, either the surrounding air or through the ground. For sufficiently intense pressure gradients, such motion may register motion or audio signals. Complimentarily, metabolic activity may yield a thermal contrast between the temperatures of a human body and the ambient surroundings.